1. Field of the Invention
The present invention relates to ink jet printing apparatus and more specifically to a system (i.e. structural configurations and operational functions) of such apparatus that provides improved self-cleaning of apparatus print head structure.
2. Description of the Prior Art
The term "continuous" has been used in the field of ink jet printer apparatus to characterize the types of ink jet printers that utilize continuous streams of ink droplets, e.g. in distinction to the "drop on demand" types. Continuous ink jet printers can be of the binary type (having "catch" and "print" trajectories for droplets of the continuous streams) and of the multi-deflection type (having a plurality of print trajectories for droplets of the continuous streams). Binary type apparatus most often employs a plurality of droplet streams while multi-deflection apparatus most often employs a single droplet stream.
In general, continuous ink jet printing apparatus have an ink cavity to which ink is supplied under pressure so as to issue in a stream from an orifice plate that is in liquid communication with the cavity. Periodic perturbations are imposed on the liquid stream (e.g. vibrations by an electromechanical transducer) to cause the stream to break up into uniformly sized and shaped droplets. A charge plate is located proximate the stream break-off point to impart electrical charge in accord with a print information signal and charged droplets are deflected from their normal trajectory. In one common binary printing apparatus charged droplets are deflected into a catcher assembly and non-charged droplets proceed along their nominal trajectory to the print medium.
The components described above (particularly the orifice plate and charge plate) must be precisely sized and positioned to achieve accurate droplet placement on the print medium. Even after this is achieved, however, significant problems are presented at each operational start-up. For example, any ink residue remaining on the charge plate from previous usage can cause shorting or improper charging of droplets. Such ink residue on the catcher assembly can affect droplet deflection or impede droplet passage to the print medium. Also, it is quite difficult to initiate the continuous droplet stream, in a stable condition along its nominal trajectory, without some initial instabilities that cause wetting of the charge plate.
Prior art solutions to the residue problem have included (i) purging the ink cavity, orifice plate and charge plates with air upon shut-down of an operational cycle; (ii) providing a nearly instantaneous negative pressure at shut-down to avoid the residue on the lower print head and (iii) introduction of cleaning solution at start-up and or shut-down. Prior art solutions to prevent unwanted wetting at start-up have included moving the lower print head charge plate structure away from its operative position at start-up or providing a rapid pressure pulse in the image bar to force an initially straight start for the jets.
These solutions are all helpful but not without related difficulties or disadvantages. For example, purging the ink system with air and/or a cleaning solution adds considerable complexity to the apparatus as well as necessitating a lengthy flushing period at start-up. Moving of the lower print head assembly's charge plate causes great potential for unaccuracy in its re-alignment with the upper print head assembly's orifice plate. The instantaneous start-up approach requires an extremely fast-actuation solenoid valve and rigid conduits and is not completely reliable in constructions where jet-to-electrode clearances are very small. Instant shut-down has similar disadvantages.
U.S. application Ser. No. 722,521, entitled "Ink Jet Printing Apparatus Having a Wet-Storage System", and filed concurrently, in the name of M. Piatt, discloses a highly useful approach for solving the above-noted problems. This approach provides a unique storage and start-up station into which the apparatus print head assembly is transported from the operative printing path. The present invention provides further improvements in the approach described in the aforementioned copending application and in particular provides structure and operational modes which effect enhanced cleaning for the lower portions of the ink jet print head assembly.